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Carbon monoxide (CO) is the main residual gas in T-ZnO field emission devices and is found to significantly decreasing the emission current. The electronic properties of the surface with CO molecule have been calculated by first-principles based on density functional theory. Carbon monoxide is weakly adsorbed on ZnO (101̅0) with adsorption energy of 1.67eV. The work function is further increased with adsorption from 4.606eV to 4.991 eV. The density of states (DOS) is minished from 118.6 to 94.34 compared to pure ZnO at Femi level. The electrons occupied at LUMO level of ZnO congregate to the CO molecule, resulting in the electrons harder to emit than the pure ZnO system. Fewer Mulliken charges were distributed near the surface, due to CO molecule repulsing the electron charges transferring from the bottom to the surface. The CO adsorption on ZnO (101̅0) surface lowers the probability of electrons emitting to the vacuum space and reduced electrons for emitting, which have a disadvantage for field emission.